magnetic disk

A flat rotating disc covered on one or both sides
with magnetisable material. The two main types are the hard
disk and the floppy disk.

Data is stored on either or both surfaces of discs in
concentric rings called "tracks". Each track is divided
into a whole number of "sectors". Where multiple (rigid)
discs are mounted on the same axle the set of tracks at the
same radius on all their surfaces is known as a "cylinder".

Data is read and written by a disk drive which rotates the
discs and positions the read/write heads over the desired
track(s). The latter radial movement is known as "seeking".
There is usually one head for each surface that stores data.
To reduce rotational latency it is possible, though
expensive, to have multiple heads at different angles.

The head writes binary data by magnetising small areas or
"zones" of the disk in one of two opposing orientations. It
reads data by detecting current pulses induced in a coil as
zones with different magnetic alignment pass underneath it.

In theory, bits could be read back as a time sequence of pulse
(one) or no pulse (zero). However, a run of zeros would give
a prolonged absence of signal, making it hard to accurately
divide the signal into individual bits due to the variability
of motor speed. Run Length Limited is one common solution
to this clock recovery problem.

High speed disks have an access time of 28 milliseconds or
less, and low-speed disks, 65 milliseconds or more. The
higher speed disks also transfer their data faster than the
slower speed units.

The disks are usually aluminium with a magnetic coating. The
heads "float" just above the disk's surface on a current of
air, sometimes at lower than atmospheric pressure in an
air-tight enclosure. The head has an aerodynamic shape so the
current pushes it away from the disk. A small spring pushes
the head towards the disk at the same time keeping the head at
a constant distance from the disk (about two microns).

Disk drives are commonly characterised by the kind of
interface used to connect to the computer, e.g. ATA, IDE,
SCSI.